The alarm echoed through the house, jolting Carolyn Shaw awake at 1 a.m. in her living room. But her husband, Gary, lay next to her in his recliner undisturbed.

“ Weeeoooo, weeeoooo, potential shock coming.”

The alarm and words were coming from the defibrillator vest her 56-year-old husband was wearing. He was unconscious, and his heart was racing at more than 200 beats per minute. He was dying.

“Weeeoooo, weeeoooo, bystanders stand back.”

Mrs. Shaw could only watch as the vest delivered 150 joules, similar to volts, that lifted her husband out of the chair and shocked his heart back to a normal rhythm.

The ZOLL LifeVest, the first defibrillator of its kind, saves more than one life a day worldwide, as it did Mr. Shaw’s, said Richard A. Packer, the chief executive officer of ZOLL Medical Corp.It can detect an abnormal heartbeat and deliver a shock, all on its own. Nearly 100,000 people around the world use the LifeVest; its use has more than tripled over the past two years, according to the company.

Mr. Shaw got his vest on Oct. 30. He had gone to his doctor the week before because his stamina was so low that he could last only three hours of a 10-hour shift as a press brake operator forming sheet metal into counters and other equipment for White Castle.

“The doctor called the next day after my appointment. The doctor himself never calls,” Mr. Shaw said. “He told me I was having a little heart attack and probably had been having them for three years.”

The attacks left Mr. Shaw’s heart so damaged that it was struggling to support his body, said Dr. Charles Noble, his cardiologist at Mount Carmel West hospital.

The measure of a heart’s efficiency — the percentage of blood leaving the heart each time it contracts — is called the ejection fraction. The amount of blood normally is between 50 percent and 65 percent for a healthy person. Mr. Shaw’s percentage was half that.The LifeVest is for people at 35 percent or less because they are most at risk of sudden cardiac arrest, which kills about 300,000 people each year, Noble said.

New guidelines require doctors to wait 40 days before implanting a defibrillator, to see whether heart function improves, said Dr. Laxmi Mehta, president of the central Ohio board of the American Heart Association.

But the first six months after heart damage are the riskiest for patients. That’s why the LifeVest can be so important, Mehta said.

Patients wear the 1-pound vest under their clothes at all times, except when bathing.

It was just three days later, on Nov. 2, that Mr. Shaw received his lifesaving shock. He woke up feeling a faint tingle from the shock, which Noble said would have felt like a bull running into his chest had he been conscious.

Mr. Shaw was rushed to a hospital, still wearing the vest. He had another heart attack at 4:20 a.m., and the vest shocked him two minutes before a crash cart with a defibrillator was brought to his room, said Mrs. Shaw, 58.

“With every minute that passes by, your survival rate decreases by 10 percent,” Noble said. “So that’s the difference of this vest, that this defibrillator is actually attached to the patient.”

The LifeVest will deliver a shock within a minute of detecting an unusual heart rhythm, and it has a 98 percent success rate on the first shock, according to ZOLL.

Noble said the LifeVest has saved the lives of at least a half-dozen of his patients. But Mr. Shaw never thought he would be one of them.

“It’s like a fire extinguisher or a seat belt,” he said. “You don’t think you’ll need it, but it’s really good to have for when you actually do.”

The father of three and grandfather of six received an implantable defibrillator the day after his two shocks.

He retired after 33 years with White Castle but hopes to be able to garden again soon.

“The vest did save my life, and more than once,” he said. “It has given me a second chance to do and enjoy the things I love.”